Self-organized transparent 1D TiO2 nanotubular photoelectrodes grown by anodization of sputtered and evaporated Ti layers: A comparative photoelectrochemical study

TiO2 nanotube (TNT) arrays directly grown on a transparent conductive support such as e.g. a layer of fluorine doped tin oxide (FTO) are highly desirable for solar energy application including dye-sensitized solar cells (DSSC) and photoelectrochemical water splitting (PEC-WS). In this paper we report on the fabrication of fully transparent TNT arrays formed by complete self-organizing electrochemical anodization of metallic Ti thin layers deposited on the FTO glass substrate by using electron beam evaporation and/or magnetron sputtering. It is clearly shown for the first time that the quality of the deposited titanium film (e.g. amount and size of pinholes) governs its adherence to the substrate and subsequently the anodization process itself. Mechanical properties of the deposited Ti films were investigated by tribology experiments. The main difference between the sputtered and evaporated titanium layers are the lower film density and considerably higher amount of pinholes of the latter. The grown TNT films were further characterized by SEM, X-ray diffraction, and UV-Vis spectroscopy. The sputtered layers overperformed the evaporated layers by 20% when applied for DSSC. An even greater difference was observed in the case of aqueous electrolyte based PEC-WS, where the sputtered layers showed five times higher activity.